Hydrocarbons, such as oil and gas, may be recovered from various types of subsurface geological formations. Such formations typically consist of a porous layer, such as limestone and sands, overlaid by a nonporous layer. Hydrocarbons cannot rise through the nonporous layer, and thus, the porous layer forms a reservoir in which hydrocarbons are able to collect. A well is drilled through the earth until the hydrocarbon bearing formation is reached. Hydrocarbons then are able to flow from the porous formation into the well.
In conventional drilling processes, a drill bit is attached to a series of pipe sections is referred to as the drill string. The drill string is rotated and, as the drilling progresses, it is extended by adding more pipe sections. Larger diameter pipes, or casings, also are placed and cemented in the well to prevent the sides of the well from caving in. Once an appropriate depth has been reached, the casing is perforated at the level of the oil bearing formation. If necessary, various completion processes then are performed to enhance the ultimate flow of oil from the formation. The drill string is withdrawn and replaced with a production string. Valves and other production equipment are connected to the well so that the hydrocarbons may flow in a controlled manner from the formation, into the cased well bore, and through the production string up to the surface for storage or transport.
As a well bore is drilled deeper and passes through hydrocarbon producing formations, the production of hydrocarbons must be controlled until the well is completed and the necessary production equipment has been installed. The most common way of controlling production during the drilling process is to circulate a drilling fluid or “mud” through the well bore. Typically, the fluid is pumped down the drill string, through the bit, and into the well bore. The hydrostatic pressure of the drilling fluid in the well bore relative to the hydrostatic pressure of hydrocarbons in the formation is adjusted by varying the density of the drilling fluid, thereby controlling the flow of hydrocarbons from the formation.
Drilling fluids, however, are used for a number of purposes. As the drill string is rotated and the bit cuts through the earth, a tremendous amount of heat and large quantities of cuttings are generated. The drilling fluid serves to lubricate and cool the drill bit. As it is recirculated back up the well bore, the drilling fluid also carries cuttings away from the bit and out of the well bore. The drilling fluid also helps stabilize uncased portions of the well bore and prevents it from caving in.
Drilling fluids may be classified based on their major liquid phase, i.e., as water to base or nonwater base fluids. Nonwater based fluids typically incorporate diesel oil, mineral oil, or a synthetic fluid such as olefin oligomers of ethylene. Water base or aqueous fluids incorporate either fresh water, seawater, or a brine. Both types of fluids may contain various other additives to impart desirable chemical and physical properties to the fluid.
Aqueous fluids have a number of significant advantages over nonwater base fluids. Typically, they do not pose as great an environmental risk as oil and synthetic fluid base fluids. Water base fluids, therefore, may be disposed of more easily and economically and are likely to create less damage in the event of accidental discharge. They also are much cheaper because their major liquid phase is water, especially when seawater is used in offshore drilling operations. Oils and synthetic fluids are more costly and must always be transported to the site. They also can present health hazards to personnel working with the fluid.
Water based fluids, however, have suffered significant shortcomings, especially when drilling through shale and other clay bearing formations. Clay tends to absorb water. As a clay bearing formation absorbs water, it swells. The swelling can result in a well bore that is out of gage, making it more difficult to insert and set casing. Swelling also can create resistance which increases the energy required to rotate the drill string or, in worst cases, causes the pipe to get stuck. Clay cuttings can clump and stick to the drill bit, what is referred to as bit balling. Also, as clayey cuttings are released into the fluid, it increases the viscosity of the fluid and makes it more difficult to pump. Swelling of a clay bearing formation also can fracture the formation and make it more susceptible to crumbling, which in turn releases even more clay into the fluid. Worse yet, the formation may become so fragile the well bore walls slough off or cave in.
Problems relating to the interaction of water with clays are especially acute in directional drilling where a well bore is being drilled along a clay bearing formation instead of simply through it. Such problems also are exacerbated because drilling commonly is conducted in an overbalanced condition. That is, the hydrostatic pressure of drilling fluid in the well bore exceeds the pressure of hydrocarbons in the formation thereby preventing the flow of hydrocarbons into the well bore. While this minimizes the to risk that a well will blow-out, a major consequence of overbalanced drilling operations is that water from the fluid may be able to penetrate clay bearing formations to a much greater extent.
Various approaches have been taken to avoid such problems with aqueous drilling fluids. Stabilization of clay particles may be achieved by controlling the charge and is electrolytic characteristics of the drilling fluid, for example, by adding various salts. Sodium chloride, calcium chloride, ammonium chloride, and potassium chloride have been used to inhibit shale swelling with the latter being perhaps the most common. Salts have been used in combination with other compounds, such as water-soluble quaternary amine-based cationic polyelectrolytes and polymers such as partially-hydrolyzed polyacrylamide which tend to encapsulate clay particles. Water-soluble organic polar compounds, such glycerol, glycol, sorbitol, erythritol, and other polyhydroxy alcohols; mixtures of polyvalent metal/guanidine complexes, cationic starches, and polyglycols; glycol compounds and derivatives thereof, and mixtures of glycols and certain organic cations and organic potassium salts have been used as well. Examples of such approaches are disclosed in U.S. Pat. No. 4,447,341 to J. Block, U.S. Pat. No. 5,342,530 to C. Aften et al., U.S. Pat. No. 5,635,458 to L. Lee et al., U.S. Pat. No. 5,925,598 to F. Mody et al., and U.S. Pat. No. 6,706,667 to C. Smith.
Another approach involves adding fluid loss control additives which serve to limit the flow of water from the drilling fluid by coating the well bore walls or building up a filter cake on the walls. The film or cake formed on the wall serves to protect permeable, hydrocarbon bearing formations from contamination from cuttings and fluid components, but it also helps to stabilize the well bore wall and to minimize contact between water and clay in the formation. Various polymers, such as acrylamido-methyl-propane sulfonate polymer and other polyacrylamides, polyacrylates, and carboxymethylcellulose, polyanionic cellulose, and other cellulosic polymers, have been commonly used for such purposes.
Starches, however, are perhaps the most common fluid loss control additive. Natural starches may be derived from corn, potatoes, and other starch producing plants, but they suffer from biodegradation and thermal instability above approximately 225° F. Excessive amounts of starch also tend to thicken the fluid excessively. Thus, various modified starches which are less susceptible to degradation by bacteria or high temperatures and provide improved rheological and thixotropic properties have been proposed for use as fluid loss control additives, either alone or in combination with other additives. Carboxymethyl and hydroxypropyl starch have been widely used. Other modified starches include pregelatinized starches, starch esters, prehydrolyzed starches, cationic starches such as tertiary and quaternary ammonium starches, various cross-linked starches. High amylose content starches have also been proposed as having relatively little effect on the viscosity and rheological properties of drilling fluids. Examples of such approaches include U.S. Pat. No. 2,786,027 to R. Salathiel, U.S. Pat. No. 3,956,141 to T. Walker, U.S. Pat. No. 4,090,968 to J. Jackson et al., U.S. Pat. No. 4,123,366 to C. Sauber et al, U.S. Pat. No. 4,422,947 to D. Dorsey et al., U.S. Pat. No. 4,652,384 to R. Slingerland, U.S. Pat. No. 4,655,942 to J. Dickert, Jr., et al., U.S. Pat. No. 4,719,021 to H. Branch, III, U.S. Pat. No. 4,822,500 to J. Dobson, Jr., et al., U.S. Pat. No. 5,658,859 to J. Burba, III, et al., U.S. Pat. No. 6,124,244 to J. Murphey, U.S. Pat. No. 6,180,571 to T. Sifferman et al., U.S. Pat. No. 6,492,305 to T. Sifferman et al., and U.S. Pub. Pat. Appl. 2004/0157748 of D. Dino.
Starches also have been used in drilling fluids as viscosifiers, as dispersants for clay solids, U.S. Pat. No. 3,417,017 to J. Kolaian et al. and U.S. Pat. No. 3,637,493 to J. Kolaian, and as gelling agents, U.S. Pat. No. 5,789,349 to A. Patel.
Despite those extensive efforts drilling through shale and other clay bearing formations with aqueous drilling fluids remains problematic. Many still regard using oil synthetic base fluids which lack significant amounts of water as the best solution and perceive the cost, environmental problems, and safety issues of oil and synthetic base fluids as lesser evils.
An object of this invention, therefore, is to provide improved drilling fluids and, in particular, aqueous drilling fluids that are more suitable for use in drilling through shale and other clay bearing formations.
It also is an object to provide drilling fluids suitable for use in drilling through shale and other clay bearing formations that may be more easily and economically formulated.
Another object of this invention is to provide drilling fluids suitable for use in drilling through shale and other clay bearing formations that may be disposed of more economically and presents fewer health and environmental risks.
Yet another object is to provide such drilling fluids wherein all of the above-mentioned advantages are realized.
Those and other objects and advantages of the invention will be apparent to those skilled in the art upon reading the following detailed description and upon reference to the drawings.